Americans don't really have a huge drinking culture like in Australia (where the age is 18) and Germany (wehere the purchase of alcohol is restricted to 16 or 18 depending on what you're buying, but you can actually drink alcohol at any age). Every time I am around Americans they always comment like all my Australian friends are a bunch of complete alcoholics. Of course, they are, but that's just how it is. Most people here over legal age drink to get drunk and make no bones about it. "Responsible" drinkers do exist but are a minority.
So I can understand how Americans can cope with waiting till 21 because drinking isn't so important over there. People in America still like to drink, but there definitely isn't the same "culture of drunkenness", that's for damn sure. If they lifted the drinking age in Australia from 18 to 21 though, I'm certain there would be rioting.